In 1932, Swiss-American astronomer Fritz Zwicky made the first attempt to measure the motion of individual galaxies within a supergalaxy, targeting the rich Coma Cluster some 320 million light years from Earth. When he discovered that the motion of the galaxies was much faster than the cluster’s visible matter could account for, he coined the phrase ‘dunkle materie’ (dark matter) to describe it. Zwicky believed that his dark matter outweighed luminous material in the Coma Cluster by around 400 times, but the discovery of intracluster gas, along with improved measurements, now suggests that dark matter accounts for approximately 85 per cent of the supergalaxy’s mass.
What’s more, dark matter seems to be widespread throughout the universe, concentrated in and around individual galaxies and clusters. This mysterious substance is not only dark but entirely transparent in all radiations, and astronomers can only measure its presence through the gravity it exerts. Perhaps the cleverest of these techniques uses gravitational lensing – the way in which large concentrations of mass distort the path of light from more distant objects beyond them. By measuring such distortions, scientists can estimate both the mass and distribution of dark matter within them, confirming that it tends to concentrate around individual galaxies.